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Radiography, echocardiography, and electrocardiography (Proceedings)

In this session we will review thoracic radiology and echocardiography with an emphasis on normal and abnormal anatomic features.

Indications

Modern imaging technology (MRI, CT, echocardiography) has dramatically improved the veterinarian's ability to diagnose cardiac and thoracic disease. However, it has not diminished the need for thoracic radiographs. Advanced imaging has not replaced thoracic radiography and thoracic radiography does not replace advanced imaging. Thoracic radiography continues to be one of the most practical and best tests for identifying parenchymal disease (congestive heart failure, pneumonia, heartworm and Neoplasia).

Remember that dyspneic animals can be extremely fragile. In some cases, it may be reasonable to treat based on suspicion of the disease for 12-24 hours to try to stabilize the patients before taking radiographs.

Limitations

Remember that thoracic radiographs can not indicate cardiac or pulmonary function. Additionally, while they may be fairly specific for cardiac enlargement, they are not very sensitive, or said another way- if significant cardiac enlargement is present it is likely real, if it is absent, it does not rule out cardiac disease. Significant breed variations are known and can complicate the interpretation of films even in normal patients. Finally it can be very difficult to interpret right ventricular enlargement patterns.

Technique for interpretation

In order to avoid misinterpretation it is ideal to examine every thoracic radiograph in a systemic fashion.

Technical quality

In almost all cases 2 views should be obtained. An exception to this would be a recheck radiograph taken within 24 hours after a thoracocentesis to determine the efficacy of a pleurocentesis. In this case it might be reasonable to just retake the lateral film.

Although different radiologists use different approaches, we generally evaluate the right lateral and a dorsoventral (DV) view. The use of a ventrodorsal approach is also acceptable, but may be more risky in animal with dyspnea or significant pleural fluid.

1. Thoracic space

The thoracic space should be evaluated for both pneumothorax and pleural fluid. Sometimes the most sensitive detection of pleural fluid is observed on a DV view in the costodiaphragmatic angles.

2. Cardiac anatomy

The cardiac silhouette should be evaluated for enlargement. If enlargement is identified, it is extremely important to then determine the specific chamber (s) enlarged. Unfortunately, breed and species differences can sometimes mislead and suggest the appearance of general cardiac enlargement even if it is not present.

The Vertebral Heart Score was developed to help reduce the confusion observed with these differences. The Vertebral Heart Score (VHS) is a method of normalizing cardiac size to body length or conformation by relating cardiac size to vertebral body (JAVMA 1995;206:194-199, Buchanan et al). The procedure is as follows:
• Measure the long axis of the heart from the ventral border of the left mainstem bronchus to the most distal ventral contour of the cardiac apex using calipers or a note card. Reposition the calipers or note card to the cranial edge of the fourth thoracic vertebrae and measure the number of vertebrae caudally to the nearest 1/10 of a vertebrae.
• In the central third of the heart, measure the maximal width or short axis of the heart at the angle that is perpendicular to the previously measured long axis. Reposition the measuring device over the thoracic spine at the cranial aspect of the fourth vertebrae and record the number of vertebrae caudally to within the nearest 1/10 of a vertebra.
• Add the 2 measurements together to equal the VHS.
• The average VHS is 9.7, most dogs are less than 10.5. Most cats are less than 8.

The Vertebral Heart Score is not very useful in cats since many cats with myocardial disease, including hypertrophic cardiomyopathy, can have normal VH scores and have very abnormal hearts.

Perhaps one of the best uses of the VHS is to study progression of known cardiac disease. For example, the VHS may be determined in cases where radiographs will be taken annually or every 6 months in patients with cardiac disease to help identify subtle size changes or different interpretations between doctors.

After evaluating for global cardiac enlargement, the specific chambers of the heart should be evaluated for enlargement. Generally we use a clock face analogy.